1. Field of the Invention
The present invention relates in general to a control apparatus for a vehicular drive system, and more particularly to a control apparatus for a hybrid vehicle drive system including an internal combustion engine and an electric motor as vehicle drive power sources, which control apparatus is arranged to reduce a shock generated upon starting of the engine when a vehicle drive mode is changed from a motor drive mode to an engine drive mode.
2. Discussion of Prior Art
In a hybrid vehicle drive system as described above, an engine-starting shock may be generated upon starting of the engine when the vehicle drive mode is changed from the motor drive mode using the electric motor as the vehicle drive power source, to the engine drive mode using the engine as the vehicle drive power source. Further, a shifting shock may be generated upon an engaging action of a coupling device of an automatic transmission which takes place to perform a shifting action. If the engine-staring shock and the shifting shock overlap each other, the vehicle operator feels a large amount of shock due to the overlap, which deteriorates a vehicle driving comfort as felt by the vehicle operator.
JP-2006-213149A discloses an example of a control apparatus for a hybrid vehicle drive system, which is arranged to solve the above-indicated problem encountered upon changing of the vehicle drive mode from the motor drive mode to the engine drive mode. In the hybrid vehicle drive system disclosed in the above-identified publication, a continuously-variable transmission portion including a first electric motor, a second electric motor and a differential mechanism and operable as an electrically controlled continuously variable transmission is disposed between the engine and an output shaft of the drive system. The continuously-variable transmission portion is provided with coupling devices which are selectively engaged and released to switch the continuously-variable transmission portion between a continuously-variable shifting state in which its speed ratio is continuously variable, and a step-variable shifting state in which the speed ratio is variable in steps. The hybrid vehicle drive system control apparatus disclosed in the publication is arranged, in view of a shifting shock of the continuously-variable transmission portion similar to the shifting shock of the automatic transmission described above, such that a selected one of an operation to switch the shifting state of the continuously-variable transmission portion and an operation to start the engine is implemented prior to the other if the switching operation of the shifting state and the starting operation of the engine would otherwise overlap each other.
The control apparatus disclosed in the above-identified publication which prevents overlapping of the engine-starting shock and the shifting shock of the continuously-variable transmission portion, and therefore prevents a large shock due to the overlap, as felt by the vehicle operator. However, the control apparatus suffers from some problems arising from its control arrangement to implement one of the switching operation of the shifting state of the continuously-variable transmission portion and the starting operation of the engine, prior to the other operation if these two operations would otherwise overlap each other. If the switching operation of the shifting state is implemented prior to the engine starting operation, for example, the engine speed is raised and started only after completion of the switching operation of the shifting state of the continuously-variable transmission portion which takes a relatively long time. In this case, therefore, a rise of the vehicle drive torque generated by the drive system is delayed with respect to a moment of rise of the drive torque expected by the vehicle operator. If the engine starting operation is implemented prior to the switching operation of the shifting state, on the other hand, the shifting state of the continuously-variable transmission portion is switched from the continuously-variable shifting state to the step-variable shifting state or vice versa, only after the engine speed is first raised and then the engine is started. In this case, therefore, the switching operation is delayed with respect to a moment of an operation of the vehicle operator to switch the shifting state of the continuously-variable transmission portion, and the rise of the vehicle drive torque generated by the drive system tends to be delayed.